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U.S. Wind StandardsModern wind design for buildings started in the U.S. with the1972 publication by the American National Standards Institute(ANSI) of ANSI A58.1, Minimum Design Loads for Buildingsand Other Structures . The 1972 standard represented aquantum leap in sophistication as compared to then-contemporary practice but was plagued with ambiguities,inconsistencies in terminology and a format that permittedmisinterpretation of certain provisions. The 1982 revision of the ANSI standard was largely free of the ambiguities andinconsistencies of A58.1-1972 and provided an innovativeapproach to the determination wind forces for componentsand cladding of buildings based on the aerodynamics of wind

pressure around building corners, eaves and ridge areas, aswell as the effects of area averaging on pressures.

Maintenance of the standard was subsequently taken over by the American Society of Civil Engineers (ASCE). ASCE7-88, Minimum Design Loads for Buildings and Other Structures , contained only minor modifications to the wind

provisions, and no changes were made in ASCE 7-93.

ASCE 7-95The next significant update of the U.S. wind provisionsappeared in ASCE 7-95—which marked the first use of 3-second gust wind speed, rather than fastest-mile windspeed, as the basis of design.

The averaging time implicit in fastest-mile wind speed isthe period it takes for a mile of wind to pass through ananemometer: typically between 30 and 60 seconds. Becauseaverage wind velocity increases as the averaging timedecreases, the design wind speed increased for the vastmajority of the country from 70 miles per hour to 90 miles

per hour (in the west, roughly in the Pacific time zone, ittypically increased to 85 miles per hour). Adjustments weremade to key coefficients in order not to end up with signif-icantly greater design wind pressures, including velocity

pressure exposure, gust effect factors, and internal andexternal pressure that included gust effects. Among other significant changes, provisions were added for wind

speed-up over isolated hills and escarpments by including atopographic effect factor in the expression for the designwind pressure.

New provisions were also added for full and partialloading on the main wind force-resisting system (MWFRS)of buildings with mean roof heights greater than 60 feet,thereby requiring consideration of wind-induced torsions.Finally, an alternate (low-rise, analytical) procedure basedon the Metal Building Manufacturers Association (MBMA)

Metal Building Systems Manual was added for determiningexternal loads on the MWFRSs of buildings having meanroof height not exceeding 60 feet.

ASCE 7-98The basic wind-speed map given in ASCE 7-98 was up-dated based on revised analysis of hurricane wind speeds,resulting in significantly lower design wind speeds in inlandFlorida.

The wind directionality factor, K d , was also introduced inASCE 7-98 to address the practical fact that wind seldomstrikes along the most critical direction of a building be-cause the wind direction changes from one instant to thenext. This had previously been accounted for by a relativelylow load factor of 1.3 on the effect of wind in strengthdesign load combinations, but the ASCE 7 Committeereceived comments that engineers using allowable stressdesign (ASD) could not factor in wind directionality. TheCommittee responded by adding K d = 0.85 to the definitionof wind pressure and adjusting the 1.3 load factor up to 1.6for strength design. Note that a load factor of 1.53 (1.3/0.85)would have maintained the status quo: the revision resultedin an effective 5-percent increase under strength design,whereas for ASD the result was a 15-percent decrease.

The Evolution of Wind Provisionsin U.S. Standards and Codes

by S.K. Ghosh, Ph.D.

46 Building Safety Journal December 2006

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In addition, the definitions of Exposures C and D werechanged slightly to allow the shorelines in hurricane-proneregions to be classified as Exposure C and new definitionsand a simplified design procedure was introduced for rela-tively common low-rise (mean roof height not exceeding 30feet), regularly shaped, simple-diaphragm buildings.

Another first in ASCE 7-98 was the organization of design provisions by method: Method 1–Simplified Proce-dure, Method 2–Analytical Procedure, and Method 3–WindTunnel Procedure. Method 2 contained two separate anddistinct procedures under the same heading—the generalanalytical procedure, applicable to buildings of all heights,and the low-rise analytical procedure, applicable to

buildings having mean roof height not exceeding 60 feet.An additional, very important, provision was introduced

requiring that glazing in the lower 60 feet of Category II, IIIor IV buildings in defined wind-borne debris regions beimpact-resistant or protected with impact-resistant cover-ing, and that such glazing which receives positive external

pressure be assumed to be an opening for design purposes.

ASCE 7-02The simplified design procedure (Method 1) was replacedin ASCE 7-02 by a slightly modified version of that givenin Section 1609.6 of the 2000 International Building Code(IBC). The IBC procedure was based on the low-rise ana-lytical procedure of ASCE 7, and its applicability is broader than that of ASCE 7-98.

ASCE 7-02 also required that a ground surface roughnesswithin each 45-degree sector be determined for a distanceupwind of the site. Three surface roughness categories were

defined, as shown in Table 1, and three exposure categorieswere defined in terms of the three roughness categories, as

SURFACEROUGHNESSCATEGORY

DESCRIPTION

B

Urban and suburban areas, wooded areasor other terrain with numerous closelyspaced obstructions having the size of single-family dwellings or larger.

C

Open terrain with scattered obstructionshaving heights generally less than 30 feet.This category includes flat open country,grandstands and all water surfaces in hurri-cane-prone regions.

D

Flat, unobstructed areas and water surfacesoutside hurricane-prone regions. This cate-gory includes smooth mud flats, salt flatsand unbroken ice.

Table 2. Exposure Categoriesof ASCE 7-02 and 7-05.

December 2006 Building Safety Journal 47

Table 1. Surface Roughness Categoriesof ASCE 7-02 and 7-05.

EXPOSURECATEGORY DESCRIPTION

B

Surface Roughness B prevails in theupwind direction for at least 2,630 feet or 10 times the building height, whichever isgreater.

CAll cases where Exposure B or D does notapply.

D

Surface Roughness D prevails in theupwind direction for at least 5,000 feet or 10 times the building height, whichever isgreater. Exposure D extends inland fromthe shoreline a distance of 660 feet or 10times the building height, whichever isgreater.

shown in Table 2. Exposure A (centers of large cities)was deleted.

The different load cases for Method 2 (the analytical procedure for MWFRS of low-rise buildings) wereclearly delineated, new pressure coefficients for thedetermination of wind loads on domed-roof buildingswere provided and provisions for calculating windloads on parapets were added. The design load cases for

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48 Building Safety Journal December 2006

the MWFRS of buildings designed by the general analytical procedure (as distinct from the low-rise analytical proce-dure) were updated as well; consideration of wind-inducedtorsion was required for all buildings, not just those havinga mean roof height exceeding 60 feet.

Finally, a low suction coefficient of 0.18 was added to thetable of roof pressure coefficients for the design of theMWFRS under the general analytical procedure for wind-ward roofs in all cases where only a high suction coefficientwas provided earlier. The intent of the new low suctioncoefficient was to require the roof to be designed for a nullor slightly positive (inward acting) pressure, dependingupon whether the building is enclosed or partially enclosed,respectively.

ASCE 7-05Several changes were made in ASCE 7-05 to the set of con-ditions that must be met by a building for its MWFRS to bequalified to be designed using Method 1. One was that therestriction that a building not be subjected to topographiceffects was omitted, with topographic effects accounted for

by including a topographic effect factor in the calculation of design wind pressure. The conditions that must be met for

building components and claddings to be designed usingMethod 1 were not changed except for the lifting of therestriction concerning topographic effects.

Under this latest edition of the standard, simplified designwind pressures and net design wind pressures can be calcu-lated for basic wind speeds of 105, 125 and 145 miles per hour. A minimum criterion is specified for estimation of

basic wind speeds from regional wind data in special windregions, explicitly indicating that the estimated speeds can

be lower than those given in ASCE 7-05, Figure 6-1.While ASCE 7-02 required Exposure D to extend inland

from the shoreline for a distance of 660 feet or 10 times theheight of a building, whichever was greater, ASCE 7-05requires Exposure D to extend into downwind areas of Surface Roughness B or C for a distance of 600 feet or 20times the height of a building, whichever is greater. In addi-tion, the multiplier of building height by which a certainterrain category has to extend in the upwind and downwind

directions of a building for qualification of any ExposureCategory has changed from 10 to 20 and other controllingdistances are rounded off to the nearest 100 feet.

A definition of eve height is now provided. Footnote 8 of Figure 6-10 (Low-Rise Analytical Procedure)—which con-cerns delineation of the boundary between windward zone

pressures and leeward zone pressures—has been clarified.Glazing in wind-borne debris regions that receives positiveexternal pressure can no longer be treated as an opening for design purposes rather than making the glazing in the lower 60 feet of a building’s height impact-resistant or protected.

Other additions include design wind loads on open build-ings with pitched or troughed roofs and provisions for rooftop structures and equipment when roof height is lessthan 60 feet.

Additional changes in ASCE 7-05 include revision of thevalues of the Combined Net Pressure Coefficient from +1.8and -1.1 to +1.5 and -1.0 for windward and leeward para-

pets, respectively; clarification of the application of the pro-visions for low-slope roofs; and an update of the designwind loads on free-standing walls and solid signs. Designwind loads on open buildings with monoslope roofs havealso been revised, and wind-borne debris requirements have

been clarified as being applicable to Method 3 (with therequirements being the same as those for Method 2).

U.S. Model CodesThe building codes of most jurisdictions within the U.S.used to be, and in some cases still are, based on one of threelegacy model building codes: the National Building Code(NBC) published by the Building Officials and CodeAdministrators International, the Standard Building Code(SBC) published by the Southern Building Code CongressInternational (SBCCI), and the Uniform Building Code(UBC) published by the International Conference of Build-ing Officials.

These codes, where still in effect, are in the process of being replaced by the IBC, first published by the Interna-tional Code Council in 2000 as part of the consolidation of the former model code groups.

The NBCANSI A58.1-1972 was adopted by reference in the 1978edition of the NBC and retained in the 1981 and 1984 edi-tions. ANSI A58.1-1982 was adopted in the 1987 editionand retained in the 1990 edition. ASCE 7-88 was adopted inthe 1993 edition of the NBC and retained in the 1996 and1999 editions.

The SBCANSI A58.1-1972 was adopted by reference in the 1977revision to the 1976 edition of the SBC, with the adoptinglanguage subsequently appearing in the 1982 edition. Winddesign using ANSI A58.1-1972 was permitted only for one-

and two-story structures provided the basic wind pressuresfrom SBC Table 1205.1 were used. Section 1206 of the1982 SBC also adopted the MBMA Metal Building Systems

Manual as an alternate standard for the design of buildingswith flat, single-slope and gable-shaped roofs with a meanroof height of 60 feet or less, provided the eave height didnot exceed the least horizontal dimension of the building.

The 1985 SBC allowed three wind design procedures.Two of the procedures were given in Section 1205, WindLoads, and the third in Section 1206, Alternate Wind Loadsfor Low Rise Buildings. The first option allowed under

Wind Provisions (continued)

(continued on page 50)

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50 Building Safety Journal December 2006

Section 1205 was the use of the provisions within thesection. The second option allowed the use of the winddesign provisions of ANSI A58.1-1982, provided the basicwind pressures of SBC Table 1205.1 were used. The table’svalues were based on the basic wind speed map depicted inFigure 1205.1, which corresponded to the 100-year meanrecurrence interval basic wind speed map given in ANSIA58.1-1972 rather than the 50-year mean recurrence inter-val map given in ANSI A58.1-1982. The alternate windload provisions of Section 1206 were permitted to be usedfor the design of buildings with flat, single-slope andgable-shaped roofs with a mean roof height of 60 feet or less, provided the eave height did not exceed the least hor-izontal dimension of the building. Section 1206 containeda corresponding basic wind speed map taken from ANSIA58.1-1982.

The 1988 edition of the SBC permitted any building or structure to be designed using ANSI A58.1-1982. In addi-

tion, Section 1205.2 of the code included provisions basedon the MBMA procedures for buildings with flat, single-slope and gable-shaped roofs whose mean roof height wasless than or equal to 60 feet but did not require that the roof eave height be less than or equal to the least horizontaldimension of the building. Section 1205.3 applied to build-ings exceeding 60 feet in height but not more than 500 feetin height, provided the roof slope did not exceed 10 degreesor was not an arched roof. Buildings between 60 and 500feet in height not meeting these limitations and all build-ings over 500 feet in height had to be designed according

to ANSI A58.1-1982. The basic wind speed map given inSection 1205 was the same as the ANSI A58.1-1982 map.

The provisions of the 1991 edition of the code wereessentially the same except that ASCE 7-88 was referencedrather than ANSI A58.1-1982. As in the updated standard,the basic wind speed map given in Section 1205 remainedunchanged from ANSI A58.1-1982.

In the 1994 SBC, ASCE 7-88 was adopted by referenceto apply to all buildings and structures. An exception con-tinued to permit the MBMA procedures, given in Section1606.2, to be used for buildings with flat, single-slope,

hipped and gable-shaped roofs with mean roof heights notexceeding 60 feet or the least horizontal dimension of the

building.The 1997 SBC wind design provisions were essentially

the same as in the 1994 edition apart from updating the ref-erenced standard to ASCE 7-95. However, the basic windspeed map given in Section 1606.2 remained unchanged(rather than adopting the ASCE 7-95 basic wind speed map

based on 3-second gust wind speed).The 1999 SBC provisions remained unchanged from the

1997 edition.

Wind Provisions (continued) The UBCThrough its 1979 edition, the wind design provisions of theUBC were based on the predecessor document to ANSIA58.1-1972: ANSI A58.1-1955.

The wind design provisions in the 1982 edition of the UBCwere based on ANSI A58.1-1972 with a simplified calculation

procedure, but important changes proposed for ANSI A58.1-1982 were incorporated. Few significant changes were madein the 1985 and 1988 editions of the code.

The 1991 UBC wind design provisions were based onASCE 7-88 with a simplified calculation procedure. Minor changes were made in the 1994 edition, and none in the1997 edition.

The IBCThe 2000 edition of the IBC adopted ASCE 7-98 for winddesign, with the exclusion of Method 1. Section 1609.6 pro-vided a simplified design procedure based on the low-riseanalytical procedure of ASCE 7-98 Method 2 and applica-

ble only to simple diaphragm buildings as defined in thecode. For qualifying residential buildings free of topo-

graphic effects, the SBCCI deemed-to-comply standardSSTD 10-99, Standard for Hurricane Resistant Residential Construction , or American Forest & Paper AssociationWood Frame Construction Manual (WFCM) were alsoallowed. The 2000 edition of the code also added an alter-native method of providing opening protection in one- andtwo-story buildings, included a conversion table betweenfastest-mile wind speed and 3-second gust wind speed, and

provided an optional design procedure for rigid tile roof coverings.

The 2003 edition of the IBC adopted ASCE 7-02 for wind

design. A simplified design procedure for simple diaphragm buildings was still provided in Section 1609.6, but it wasnow very close to ASCE 7-02 Method 1 because—as previ-ously noted—ASCE 7-02 employed the simplified design

procedure given in 2000 IBC Section 1609.6 with somemodifications. Qualifying residential buildings free of topo-graphic effects could still be designed using SSTD 10 or WFCM. The alternative method of providing opening pro-tection in one- and two-story buildings, the conversion table

between fastest-mile wind speed and 3-second gust windspeed, and the optional design procedure for rigid tile roof

coverings remained essentially unchanged.The 2006 IBC has adopted ASCE 7-05 for wind design.

Although simplified wind design is no longer in the code,it is permitted by reference to the ASCE standard andqualifying residential buildings free of topographic effectscan still be designed using SSTD 10 or WFCM. The alter-native method of providing opening protection in one- andtwo-story buildings is also retained in a modified form aswell as a revised conversion table between fastest-mile windspeed and 3-second gust wind speed. The optional design

procedure for rigid tile roof coverings remains unchanged.(continued on page 52)

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Changes fromANSI A58.1-1972 to ASCE 7-05Of all the changes from ANSI A58.1-1972 through ASCE 7-05,only a few are less conservative from a design perspective.

The first of these is the adoption of the low-rise analytical pro-cedure in ASCE 7-95 as an alternative design approach for theMWFRS, which can reduce design wind pressures significantly.While generalizations can be difficult because so many variablesinfluence the determination of design wind pressures for a spe-cific building, use of the alternate procedure can result in the totalwind load being approximately 30- to 35-percent less than would

be calculated using the primary procedure. In areas where the basic wind speed is low, the relative lack of conservatism of thelow-rise procedure is mitigated somewhat by the requirement thatall MWFRS be designed for a minimum pressure of 10 pounds

per square foot applied to the area of the building projected ontoa vertical plane. It could be argued, however, that this provisionis often overlooked by practitioners and is not sufficientlyenforced by local jurisdictions.

It is also worth noting that the 3-second gust speed map of ASCE 7-95 was prepared from data accumulated by the NationalWeather Service rather than being converted from the ASCE 7-93fastest-mile wind speed map. While in most areas the 70 mile-

per-hour fastest-mile wind speed, for example, became a 3-second gust speed of 85 or 90 miles per hour, in certain areas suchas Denver, Colorado, it remained virtually unchanged. In prac-tice, this meant that design wind pressures in those areas wentdown when ASCE 7-95 was adopted, even while using strengthdesign, with the rounded-up load factor of 1.6 incorporated.

Another considerable change was the introduction of the direc-tionality factor, K d , in ASCE 7-98. This led to a rounding-up of the wind load factor from 1.53 to 1.60 in strength design anddecreased the design wind forces in Allowable Stress Design,which is widely used for structures made of materials other thanconcrete.

The final change in the direction of reduced conservatism wasthe redrawing of the basic wind speed map in ASCE 7-98, whichdecreased the basic wind speeds in inland Florida. It may beargued, however, that when U.S. National Weather Service dataindicates that a change is warranted, the ASCE 7 Committee haslittle grounds for dispute.

By and large, the changes from ANSI A58.1-1972 throughASCE 7-05 have not been consistently in the direction of lower or higher design wind pressures. If there is a demonstrable trend,it is toward more complexity in wind design. ◆

S.K. Ghosh, Ph.D. , is President of S.K. Ghosh Associates, Inc.,which provides code-related seismic and specialized structural consulting services to clients in the U.S. and abroad, and

Adjunct Professor of Civil Engineering at the University of Illinois at Chicago.

Figure 1. Plan of example concrete building.

Figure 2. Elevation of example concrete building.

52 Building Safety Journal December 2006

Wind Provisions (continued)

Dr. Ghosh served for many years as Director of Engineering Services, Codes and Standards for the Portland Cement Association and has written exten- sively on wind and seismic engineering issues.

Much gratitude is expressed to Jim Messersmith andSteve Skalko of the Portland Cement Association and Susan Dowty of S.K. Ghosh Associates Inc. for theircontributions.

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Given a concrete building in suburban Los Angeles,California—which has a 3-second gust wind speed of 85miles per hour—in exposure Category B and a plan andelevation as illustrated in Figures 1 and 2, respectively,we

may calculate design wind forces using both ASCE 7-05Method 2,which has been adopted by the 2006 IBC, andthe wind design procedure of the 1997 UBC, which is asimplified version of that given in ASCE 7-88 developedas a joint effort by the Structural Engineers Association of California (SEAOC) and the Structural Engineers Associa-tion of Washington (SEAW).

As illustrated in Table 3, although the calculationsinvolved in the two cases were not comparable, withthe ASCE 7-05 design being considerably morecom-plex (even with the different load cases given inFigure 6-9 of ASCE 7-05, other than Load Case 1, notcon-sidered), the UBC procedure produces slightlyconservative results. The primary reason for the highercomplexity is that the simplifications made bySEAOC/SEAW to the provisions of ASCE 7-88 are notallowed using ASCE 7-05 Method 2.

Also, as outlined in the main article, many complexi-ties have been added to the wind design provisions of ASCE 7 between the 1988 and 2005 editions. Forexample, because the building under consideration isflexible, ASCE 7-05 requires that the gust effect factor

be calculated—involving a large number of complexequations that took an experienced engineer over 90minutes to complete. Ironically, the factor turned out tobe 0.87, compared with the 0.85 value prescribed for

rigid buildings.While generalization is not reasonable based on just

one example, this demonstrates the potential benefitof including a simplified design procedure in the IBC,with appropriate restrictions.The 1997 UBC design pro-cedure cannot be used as-is because it is based onfastest-mile wind speed, which is no longer recordedby the National Weather Service, and other aspects of the procedure should also be updated to reflect thelatest research and technology.

A simplification of the analytical procedures of ASCE

7-98 and 7-02 has been under development by SEAWfor some time and was recently published by theApplied Technology Council as SEAW RSM-03, SEAW’sHandbook of a Rapid-Solutions Methodology for Wind Design, along with SEAW/ATC-60, Commentary onWind Code Provisions. This procedure does not yetappear to be ready for codification and the mosteffective way of finalizing the update would bethrough collaboration among state structural engi-neers associations. ◆

FLOOR LEVELWIND FORCES (plf)

2006 IBC/1997 UBC1997 UBC 2006 IBC

R 235 214 0.91

20 314 286 0.91

19 310 284 0.92

18 306 281 0.92

17 302 278 0.92

16 298 275 0.92

15 293 272 0.93

14 290 269 0.93

13 285 265 0.93

12 282 262 0.93

11 277 258 0.93

10 272 254 0.93

9 267 249 0.93

8 261 244 0.94

7 253 239 0.94

6 247 233 0.94

5 239 226 0.94

4 229 218 0.95

3 218 208 0.95

2 243 233 0.96

Table 3. Comparison of computed wind forces for example building.

1997 UBC/2006 IBC Design Comparison

December 2006 Building Safety Journal 53